This invention relates to an image display apparatus and method for applying dither halftoning (dithering) processing to an externally applied video signal.
Graphics cards mounted in computers are now being produced with high resolution, with the capability of displaying multiple colors and in a wide variety of types. For example, graphics cards capable of supporting as many as 16,700,000 displayed colors are now available on the market. In terms of the types of graphics cards available, these include graphics cards whose numbers of horizontal display dots and vertical display dots are 640.times.480 dots, 800.times.600 dots, 1024.times.768 dots, 1280.times.1024 dots and 1600.times.1280 dots, etc. Thus, resolution is steadily increasing. In addition, the video signals output by these various types of graphics cards often differ subtly in terms of timing and level.
In a case where these video signals are received and displayed on, say, a flat-panel display, there are instances where the flat-panel display does not have the capability to display the number of display colors represented by the received video data. In case of such a flat-panel display, the number of apparent display colors is enlarged by successively subjecting each of the pixels of the input video signal to dither halftoning.
However, the following problems arise in the example of the prior art set forth above:
(1) The transfer frequency of the input video signal is raised by the improvement in the resolution of the graphics card. Accordingly, when the input video signal is transferred to a dithering circuit, a technique is adopted in which the input video signal is transferred to the dithering circuit after being converted to a parallel signal (a plurality of synchronized serial signals) by a demultiplexer in order to take into account reflection of the signal and unnecessary radiation. This makes it necessary to carry out the dither halftoning process in parallel fashion for each of the serial signals. On the other hand, in case of a graphics card having a low resolution, the input video signal is not converted to a parallel signal by a demultiplexer in view of the need for better performance and to hold down cost. If the dither threshold values with which the input signal is compared are made of fixed values when it is attempted to obtain an optimum video signal commensurate with the capabilities of the graphics card and display, the output data resulting from halftoning by the dithering circuit differ, even if the input video signal is the same, depending upon whether the input video signal is subjected to dither halftoning processing in parallel after the conversion by the demultiplexer or without being converted to the parallel signal by the demultiplexer.
(2) In a case where only the part of the data to be displayed is captured from the input video signal received from the graphics card, changing the scope of the area captured results in a change in the dithered data because the dither threshold values differ depending upon the scope of the area captured.
(3) Since gamma correction values and signal levels differ depending upon the graphics card, it is necessary to correct the input video signal before dither processing is executed. If the dither halftoning circuit is constructed as an ASIC, the circuit is large in scale and problems arise in terms of cost and the evolution of heat.
(4) The numbers of colors capable of being displayed by a flat-panel display and the configurations of the color filter differ depending upon the panel. Therefore, if the size of the dither matrix and the threshold values are fixed, optimum dither halftoning cannot be executed if the panel is replaced by another. Further, the display dots and lines of a graphics card change depending upon the display mode of the graphics card. Hence, if the dither matrix is fixed, the proper color tones cannot be displaced when the display mode is changed.
(5) In a case where the input video signal is an interlaced signal such as a television signal, the threshold values used in dither halftoning are the same for both odd- and even-numbered fields and the number of apparent colors capable of being displayed as halftones decreases.